It was loud enough to be anoying! One of the many reasons I dumped the Avalon!
On the Avalon, you could open the hood, pull the wipers to the resting position away from the glass, and operate the wipers. You could then hear and feel the noise from the wiper motor. However, due to the hood design of the ES (at least on my 97), you can't pull the wipers away with the hood open. I guess you will have to disconnect the wiper arm linkage to verify my suspisions.
Good Luck,
Gary